Interpretive Summary: Sheath blight is a major disease of rice in the USA and worldwide. Some rice cultivars, found in various rice growing areas of the world, have been identified as moderately resistant to sheath blight but no resistant cultivars have been discovered to date. Wild relatives of rice (Oryza species) are another source of novel sheath blight resistance genes. Two of these species, Oryza rufipogon and Oryza nivara, are the ancestors of cultivated rice. In other words, these are plant species that humans domesticated in prehistoric times to select what is now known as cultivated rice (Oryza sativa). Rice wild relatives are often a source of new pest resistance genes that can be incorporated into our currently grown rice varieties to improve pest resistance. Most all testing of rice cultivars for their reaction to the sheath blight disease is done in the field. Because the rice wild relatives drop seed very easily (shatter) and cannot be grown in the field, a different method is needed to evaluate them for sheath blight resistance. Three different methods were used to evaluate the reaction of 73 Oryza species accessions originating from different areas of the world to the sheath blight disease. Seven accessions were identified as moderately resistant to sheath blight. These accessions represent the species Oryza barthii, Oryza meridionalis, Oryza nivara, Oryza officinalis, and Oryza nivara/Oryza sativa. Efforts are now underway to further identify the sheath blight resistance gene(s) and incorporate this resistance into rice varieties adapted to the United States. These U.S. adapted, sheath blight resistant lines will eventually be used by rice breeders to develop new and improved rice varieties that are resistant to the sheath blight disease.

Technical Abstract:
Oryza species, wild relatives of cultivated rice (O. sativa), may contain novel resistance genes to sheath blight, caused by Rhizoctonia solani Kühn, that could be used to enhance resistance to this important disease in commercial rice. Suitable greenhouse screening methods are needed to identify resistance in wild Oryza species as there are significant limitations to screening efforts under field conditions. Three screening methods were evaluated with 73 Oryza genotypes. For the micro-chamber method, 4-week-old seedlings were inoculated with a potato dextrose agar plug containing mycelia, covered with a 2-liter soft drink bottle, and rated one week after inoculation. A detached-leaf method involved placing an agar plug containing mycelia on the abaxial surface of a leaf section that was cut from a 5-week-old plant and placed on moist filter paper in a petri dish under constant light, then evaluated after 72 hours. For the toothpick inoculation method, toothpicks colonized with mycelia were placed in the leaf collar region of plants at the panicle initiation stage, plants were placed in a growth chamber, and disease symptoms evaluated after seven days. The micro-chamber method gave a more uniform and reproducible response, was easier to use under greenhouse conditions, and was better correlated with disease evaluations of the reference cultivars grown under field conditions. The micro-chamber and detached leaf data were subjected to a cluster analysis, and seven Oryza species accessions were identified as moderately resistant, similar to the most resistant reference cultivars.